The present invention relates to an adaptive temperature control system in which the cycle time and duty cycle of a temperature controlling device is adjusted automatically to be within a predetermined range.
In many cooling systems such as refrigerators, a thermostat is located in heat conduction relationship with a discharge air duct or the discharge air stream in order to sense discharge air temperature, thus exposing the thermostat to large temperature swings. The thermostat has a predetermined amount of hysteresis, and switches the compressor on and off in cycles which vary greatly dependent upon the operating environment of the refrigerator. Low ambient temperatures generally produce a compressor cycle having a very short run period and long off periods, which cycles may result in undesirable temperature stratification in the refrigerator. High ambient temperatures generally cause the compressor to run for very long on periods followed by very brief off periods, leading in some cases to short cycling which can cause compressor damage or overload.
The refrigerator temperature control must function at various ambient temperature and for various thermostat settings. Conventional thermostat controls have fixed set points determined by the amount of hysteresis present, and considerable design time is required to adjust the thermostat to give the desired degree of temperature regulation. The temperature that the thermostat will maintain may be dependent on other controls in the refrigerator, for example, varying the freezer damper may change the temperature in the above-freezing compartment.
It is known that long compressor cycles provide the best operating efficiency and maximum component life. Thermostats and other temperature control systems have included restart delay devices to provide a minimum off time before the compressor is restarted, and a minimum on or run time by other delay devices.
In some refrigeration control systems, a percentage timer has been used to control the compressor duty cycle, in which the timer allows manual adjustment of the run time and manual adjustment of the cycle time. This allows a user to set a preferred duty cycle and cycle time for the compressor, but does not automatically adapt these variables in response to changing conditions. Until the user intervenes, the control provides a fixed run time and a fixed cycle time.
Various temperature control systems have attempted to automatically adjust to certain changing conditions. It has been known to control the duty cycle by sensed temperature, with manual adjustment of the cycle time. In one digital control proposed for a refrigerator, the run time has been controlled by sensed temperature while a fixed off time has been provided by a counter which increments time intervals as measured by the fixed frequency of the connected power line. Of course, the widely-used thermostat system controls the on time and the off time in response to sensed temperature.
All such temperature controls have significant shortcomings. Where short cycling is prevented by providing a fixed time delay device, the control cannot fully adapt to changing ambient temperatures, heavy or minimum usage, and the like. Where fixed time delays are not provided, short cycling can occur and result in damage to the refrigerator components. These prior designs tend to be energy-inefficient in that the cycle patterns which are automatically varied are controlled only by sensed temperature. A true adaptive temperature control in which the cycle pattern and duty cycle are varied in accordance with a changing environment has not been possible.